In the production of thin sheet metal used in such products as automobile bodies and household appliances, it is essential that the thin sheet metal exhibit the most attainable uniformity in thickness or gauge. When the thin sheet metal known by those skilled in the art as "strip" is stamped or pressed, it is extremely sensitive to any variations in the strip gauge.
Gauge variation in the strip causes stamping or pressing defects resulting in holes, tears, ripples or other flaws that often mandate scrapping the finished product.
The production of thin sheet metal or strip is most commonly accomplished by using two sequential manufacturing processes. The first step occurs in two phases. The first phase consists of reheating an object such as a slab to a temperature sufficient to produce pyroplasticity in the object. The second phase occurs in the hot strip mill. The hot strip mill process reduces the thickness of a large rectangular object such as a slab that has been heated to a temperature sufficiently high to allow pyro-plastic deformation. The slab typically has dimensions of about 9-10" thickness.times.30-84" width.times.30-40' length, and is usually heated from ambient temperatures (often 50.degree.-90.degree. F.) to 2000.degree.-2500.degree. F. depending on the type steel being processed. The slab may enter the reheat furnace at a higher temperature than the ambient temperature noted above if it has retained heat from prior processing steps.
Once the object such as a slab has been heated in the furnace, it is continually passed through a number of rolling mill stands positioned along the length of the mill. Each of the stands contains at least two large rotating cylindrical rolls between which the slab is passed and squeezed to reduce the thickness of and elongating the slab. The rolling mill process is by analogy closely akin to the well known washing machine wringer. The slab is squeezed between the rolls, and with each successive pass through each mill stand, the slab is incrementally reduced in thickness. The ultimate strip thickness is frequently in the range of 0.080-0.250" while the length is increased to as much as 4000 feet.
The strip is generally stored as a tightly rolled coil which is banded after cooling for ease of handling. Often the coil is then (1) annealed to result in a "dead" soft material, (2) pickled to remove foreign material such as residual scale and (3) further processed by cold rolling to achieve the desired final product gauge. The product gauge for certain uses is often in the range of 0.008-0.020".
The slabs are usually heated in a reheat furnace prior to processing into hot strip and plate products. It is this portion of the entire process of production to which the invention relates.
The furnace is generally either a pusher type or a walking beam type furnace. In both types of furnaces, a network of water-cooled piping commonly known as a skid system is used to support the slabs during the heating cycle. The skid system is usually water-cooled to maintain its mechanical strength at temperatures frequently in the range of 2450.degree. F.
In pusher type furnaces, each skid comprises a horizontal, water-cooled pipe that is equipped with a steel or high temperature alloy wear bar affixed to the top of the skid pipe. The skids are orientated substantially parallel with the direction of the slab as it is pushed through the furnace. Larger pusher type furnaces often utilize 6 to 8 skids with 4 feet to 8 feet spacing between the skids. These skids are themselves supported by a number of horizontal pipes located below and orientated perpendicular to the skids. These support pipes are known by those skilled in the art as cross pipes and are themselves supported by vertical pipes that support the entire skid system and the slab riding atop the wear bars.
In the pusher type furnace, the slabs are usually mechanically or hydraulically forced into the furnace along the skids one after the other so that with each new slab that is pushed into the furnace, the other slabs that are already in the furnace are themselves pushed along the skids.
Those skilled in the art realize that heat is conducted from the slab, through the wear bar and into the water cooled pipe. Heat transfer is also lost to the slab due to the shadow cast on the underside of the slab by the skid. Newer pusher furnaces utilize an offset skid system to reduce thermal conduction from and to increase the thermal radiation upon the slab in order to even out the cold spots created by those effects.
Walking beam furnaces differ from pusher furnaces in part by eliminating the pushing mechanism. A walking beam furnace is equipped with auxiliary moveable skids that are usually located midway between the stationary skids. The continuous wear bars may be replaced with intermittent, spaced apart wear buttons that support the slab. These "walking skids" normally reside 4-6" below the stationary skids in order not to interfere with the radiant heat transfer from the burner blocks to the slab when the slab is at rest. The slab is transported through the furnace by the walking skids which are articulated by a combination of vertical and longitudinal linkages that (1) elevate the slab above the stationary skids, (2) move the slab load forward toward the discharge end of the furnace, (3) lower the slab onto the stationary skids and (4) then retract to the original position below the stationary skids. This sequence is repeated at calculated intervals to accommodate the desired pacing rate of the rolling mill.
Walking beam furnaces are emerging as a preferred type of furnace for heating steel slabs because: (1) damage to the bottom surface of the slab from scouring while being pushed through the furnace is reduced; (2) the slabs can be spaced with gaps between the adjacent slabs to increase the radiant heat transfer to those surfaces thereby reducing the heating time and improving fuel efficiency; (3) areas of lower temperatures known as cold spots are less intense because of the reduced area of slab contact with the non-continuous and smaller wear buttons that support the slabs; and (4) cold spot intensity is further reduced due to the periodic residence cycle and time that the slab spends in alternating contact between the stationary and walking skids.
Those skilled in the art understand that cold spots also occur in a slab in a walking beam furnace in part because of conduction of heat from the slab through the wear button into the water-cooled pipe and the shadow effect of the skids located between the heat sources or burner blocks of the furnace and the slab.
Cold spots occur more from the shadow effect upon the lower surface of the slab than from conductive heat transfer through the wear bar or wear button.
The prior art is replete with attempts to reduce the cold spots on the bottom surface of the slab or other object being heated within the furnace. For example, U.S. Pat. No. 4,936,771 issued to Sidwell discloses a structure for focusing the burners on top of the slab to a point on the upper surface of the slab directly opposite the cold spot generated below. U.S. Pat. No. 3,642,261 to Laws discloses a triangular skid pipe having alternating wear bars for distributing the cold spots. U.S. Pat. No. 4,391,587 to Murakami, et al discloses a connecting device which minimizes the number of vertical support pipes interfering with the flame pattern. Finally, U.S. Pat. No. 4,492,565 to Feroldi discloses a structure which actually turns a billet or bloom over 180 degrees at least once so that the cold spots are exposed upwardly. All of these devices as well as the prior art in general have not resolved the long felt need to reduce the shadow effect and the resulting cold spot upon the object to be heated.
The present invention directly relates to reducing the shadow effect caused by the skids on the slabs in both walking beam furnaces and pusher furnaces thereby increasing the thermal radiation heat transfer from the heat source to those slabs.